Well, I am using now a simple circuit. I have connected to the tip and
ring
  a diodes bridge to fix the positive and negative references.
  For obtain the isolated signal corresponding to line presence have a 68K
  resistor in a diode of a opto-isolator. I need polarize the opto-isolator
  without to affect the telephone line. If I use low value resistors the
  telephone line is "answered" (dropped to a low voltage signal) and if I
use
  big value resistor I can't match the polarization of opto-isolator.


  How can I fix it?

  (sorry by bad english ;-) . I hope that you understood me.)



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